A dry electrographic image such as an electrophotographic image is typically produced by initially forming an electrostatic latent image on a primary imaging member. This image can be formed, for example, by first charging a photoconductive element included in a primary imaging member, then discharging selected portions of that element using optical exposure or an electronic means of exposure such as a laser scanner or an LED array. The resulting electrostatic latent image on the photoconductive element is developed by bringing it into close proximity to an appropriate developer comprising marking or toner particles, which are deposited onto the latent image to convert it into a visible image. The resulting visible image is then transferred to a receiver sheet such as paper using a variety of techniques such as applied heat or pressure, but most commonly by the application of a suitable electrostatic field to urge the toner towards the receiver. After transfer, the image is permanently fixed on the receiver, typically using heat or pressure or a combination thereof to soften the toner comprising the visible image, causing it to be fused and thereby permanently affixed to the receiver. The primary imaging member from which the image has been transferred is then cleaned and made ready for subsequent imaging.
Color images are generally produced by first producing electrostatic latent images corresponding to the primary color separations of the image. For example, to produce a full-color image, cyan, magenta, yellow, and black separations are produced, preferably on separate frames of the primary imaging member. A single frame can be used for all the separations, in which case it is desirable to transfer each separation image after development to a receiver. It is possible, though less desirable, to develop all the images sequentially on the same frame of the primary imaging member and then transfer the entire image to the receiver in one pass. The individual visible separation images are then transferred in register to the receiver.
It is often desirable to first transfer a toned image from the primary imaging member to an intermediate transfer member by the application of a suitable electric field. Images corresponding to the toned separations can be transferred, in register, to the intermediate transfer member and subsequently transferred to the receiver by application of a second electric field to urge the toned image from the intermediate transfer member to the receiver. Alternatively, the separation images can be transferred to the intermediate transfer member and then to the receiver, with the final registration occurring on the receiver. It should be noted that, where reference to four colors is made in this discussion, more or fewer colors can be straightforwardly employed. The intermediate transfer member can comprise either a drum or a web and is preferably a compliant member, as is known in the art.
Color printed images produced on xerographic devices have found many usage in both and commercial and consumer applications. One application that is increasingly becoming more important is the photo printing market. In order to duplicate the image quality that can be achieved with silver halide process, however, the image quality of the toner based electrophotographic process needs to be further improved. For this reason, toner manufactures are continuously trying to decrease the size of the marking particle. Previously, color electrophotographic printers used toner particles which were in the 10-12 microns range. More recently, the color toner particle size typically used is in the 6 to 8 microns range in an attempt to meet the increasing higher image quality needs. There are many factors that make it extremely difficult to use even smaller toner particles in printers. One of the main reasons for such difficulties is the inability of the existing conventional Melt Pulverized Toner (MPT) manufacturing processes to make a smaller toner in an economical manner.
In the conventional Melt Pulverized Toner (MPT) process, the desired polymeric binder for toner application is produced independently. Polymeric binders for electrostatographic toners are commonly made by polymerization of selected monomers followed by mixing with various additives and then grinding to a desired size range. During toner manufacturing, the polymeric binder is subjected to melt processing in which the polymer is exposed to moderate to high shearing forces and temperatures in excess of the glass transition temperature of the polymer. The temperature of the polymer melt results, in part, from the frictional forces of the melt processing. The melt processing includes melt-blending of toner addenda into the bulk of the polymer.
The melt product is cooled and then typically initially ground to a volume average particle size of from about 18 to 50 micrometers. It is generally preferred to first grind the melt product prior to a specific pulverizing operation. The grinding can be carried out by any convenient procedure. For example, the solid toner can be crushed and then ground using, for example, a fluid energy or jet mill, such as described in U.S. Pat. No. 4,089,472, and can then be classified in one or more steps. The size of the particles is then further reduced by use of a high shear pulverizing device such as a fluid energy mill to yield toner particles as small as about 6 microns. But as further reduction in toner particles are made, the energy requirements as well as the grinding times increase rapidly in a relationship which is proportional to the amount of surface area which is created. Further, as smaller toners are produced, the amount of toner fines generated during the pulverized also increased. These fines have to be removed from the distribution using typical classification processes prior to the toner being used. As a result, the yields become increasingly lower as smaller toners particles are produced. This all leads to increase in the toner cost. Therefore, typical MPT processes are not practical for making small toner particles.
There are additional reasons for producing smaller toner size marking particles. As the size of the toner is decreased, the amount of toner used for printing also is reduced. This leads to improved cost performance as well as reduced image relief. Further, the fusing becomes easier as the toner stack is reduced.
As an alternate approach to making toners particles, Chemically Prepared Toners (CPT) is becoming increasingly more popular. One of the main advantages with chemically prepared toner is their ability to produce smaller particle size toners with narrow size distribution. In general, the method of chemically produced toners involves growing small particles till the desired particle size has been achieved. Although the term CPT is used to describe all non-melt pulverizing methods of producing toners, the various methods used to make such toners are very different. Among the several methods for making chemically prepared toner are the polymer suspension, suspension polymerization, and Emulsion Aggregation (EA) processes, which are very well known in the art.
The developer employed in electrophotographic printing comprises marking or toner particles and preferably further comprises magnetic carrier particles in a so-called two-component developer, which is generally used in a magnetic brush, known in the art. In addition, the developer can include a third component comprising particulate addenda of submicron size, for example, silica, strontium titanate, barium titanate, titanium dioxide, various polymeric particles. These addenda are typically employed to control flow, enhance transfer, and control toner charge-to-mass characteristics. The developer may also comprise other materials such as charge agents.
It is important in electrophotographic development that the toner be electrically insulating. If it is not, the absolute value of the toner charge-to-mass, referred to hereafter simply as “toner charge-to-mass,” can become so low that mechanical agitation at the development station causes the toner to separate from the developer as a dust cloud, whose deposition on the primary imaging member results in unacceptable background in the final print. In addition, the airborne toner can be deposited on other surfaces such as those of the charging device, causing contamination that adversely affects the operation of the device, resulting in lost productivity and possibly requiring an expensive service call. Such problems are particularly troublesome at magnetic core development stations, especially those in which the core rotates, referred to as the SPD process, as described in Miskinis, IS&T Sixth International Congress on Advances in Non-Impact Printing, pp. 101-110. In such stations the magnetic core imparts significant agitation to the developer, thereby inducing significant dusting if the toner has too low a charge-to-mass.
The electrostatic transfer field for transferring the toned image to either the intermediate transfer member or the receiver can be accomplished in a number of ways known in the art, most frequently through the use of either a biased roller or a corona charger. A compliant intermediate transfer member can comprise the biased roller.
Although many receivers are known in the art, including transparency stock, cloth, and metal, paper is most commonly employed as the receiver. It is generally desirable that the transfer member, intermediate transfer member, and receiver have finite resistivities in order to establish the electrostatic transfer field. Furthermore, to ensure successful toner transfer, it is necessary that the toner particles bear an electric charge that is maintained throughout the transfer process. The electrostatic force urging the toner to transfer is the mathematical product of the charge on the toner and the applied electrostatic transfer field. If the toner loses its charge, or worse, if the sign of the charge changes during the transfer process, the toner would fail to transfer.
To prevent toner from discharging, the toner must be electrically insulating, with no electrically conducting components residing at the toner particle surface, where they could contact a second electrically conductive material such as paper, fabrics, metals, etc., during the transfer process. Were this to occur, charge could travel from a conducting component at the toner surface to the second conductive material under the influence of the electric field, causing the toner to reach an equipotential state with the second material, for example, a paper receiver. Under normal relative humidity conditions, paper is fairly electrically conductive. Charge would bleed from the toner to the paper, ultimately reaching the potential of the paper. Under this circumstance, the toner would be more attracted to the transfer member than the paper receiver, thereby preventing toner transfer. The toner could also lose charge in the development station by contacting carrier, other toner particles, or metallic components of the station.
Although the polymer binder included in the toner is insulating, electrically conducting agents, for example, electrically conducting pigments such as carbon are frequently incorporated into toner particles. Carbon is a preferred pigment for black toner because it is inexpensive and non-fading, but it is also electrically conductive. This conductivity of carbon generally does not present a problem if it is dispersed into a molten polymer binder to form a solid block of pigment-binder material, from which toner particles are produced by grinding and classifying. However grinding and classification techniques are disadvantageous for the production of toner particles of uniform size distribution and small diameter, i.e., mean volume weighted diameter less than 8 μm, as measured by devices such as a Coulter Multisizer, available from Coulter Electronics, Inc. For the production of such toner particles, colloidally stabilized limited coalescence (LC) suspension processes that entail dissolving either the polymer comprising the toner binder (“polymer suspension”) or the monomers that combine to form the polymer binder (“suspension polymerization”) in an organic solvent, and dispersing appropriate additional toner components such as the pigment particles in the solution, are useful. Colloidally stabilized suspension processes useful in the practice of the present invention are described in, for example, U.S. Pat. Nos. 4,833,060; 4,835,084; 4,965,131; and 5,133,992; the disclosures of which are incorporated herein by reference.
In colloidally stabilized suspension processes, which are carried out in a mixture of water and a hydrophobic organic phase, fine hydrophobic particles such as silica, titania, various latices, etc., prevent the formation and separation of macroscopic hydrophilic and hydrophobic phases. If desired, the particles that limit coalescence can be removed by such processes as dissolution in strong alkalis, etc. Throughout this disclosure, toners formed by dispersing pigments and hydrophobic solutions of polymers or monomers in water will be referred to as LC toners. Although LC toners formed in this manner generally charge well, black LC toners, defined as LC toners that include carbon as the pigment, do not. Specifically, black LC toners tend to display an undesirably low charge-to-mass. Consequently, the force applied to the toner to urge it from the transfer member may be insufficient to overcome those forces holding the toner to the member. Moreover, although it might be expected that transfer would improve with increasing transfer voltage until air breakdown occurs, transfer that appears satisfactory at low voltages may unexpectedly achieve an undesirably low maximum prior to decreasing with increasing transfer voltage. Also, black carbon may flocculate in LC processes, leading to less than desired covering power.
In U.S. Pat. No. 5,118,588, the disclosure of which is incorporated by reference herein, there is described a process of making chemically prepared toners by which a pigment surface can be rendered hydrophobic by reacting the hydrophilic pigment particles with a relatively low weight percent of additives that contain some functional groups. Although high transfer efficiency is demonstrated for the resulting toner, the pigment dispersion in the individual toner particles may not be uniform as evident from the TEM cross-sections, which may result in lower than desired printing densities and covering power for the toner. Publication No. US2001/0055722, the disclosure of which is incorporated by reference herein, discloses use of LC toners comprising carbon black pigment of specified BET value and use of submicron particulate surface treatment to provide high transfer efficiencies. Although transfer efficiencies may be improved, pigment dispersions in individual toner particles again may not be as uniform as desired. When smaller toners particles are desired which are capable of delivering high optical density, these approaches may not be sufficient. Further, if more carbon is added to increase the optical density, the approach leads back to the issue of lowering the charge/mass and reduced transfer efficiency.
Thus there is a continuing need for black toner compositions, and in particular relatively small sized black toner compositions, that provide high charge/mass and high transfer efficiency, especially from the intermediate transfer member of an electrophotographic apparatus to a paper receiver, as well as providing good optical densities and covering power. This need is met by the toner composition and process of the present invention.